1. Field of the Invention
This invention relates to an optical system for observing or photographing therethrough the fundus of an eye, and more particularly to an optical system for effecting the correction of the diopter of an eye to be examined.
2. Description of the Prior Art
For the purpose of observing or photographing an eye fundus and correcting the diopter of the eye, there are known the inner focus type and the outer focus type devices. In a device based on the so-called outer focus type method wherein the position of the fundus image is varied by the diopter of the examiner, a telecentric system is used for the optical system to eliminate variation in image height and the construction is relatively simple. However, the position of the image is varied to thereby vary the length of the lens barrel and this is not preferable. For example, in case of a so-called fundus camera which is chiefly directed to photographing, the camera portion must be movable by means of bellows or the like and this leads to a larger size of the entire device and accordingly to poor operability. In case of a portable eye examining device, compactness and light weight must be given priority and the outer focus type is not appropriate. On the other hand, unlike the outer focus type, in the inner focus type wherein the position of the fundus image is kept fixed, the optical system itself is complicated making it difficult to provide a compact and light-weight device.